1. Field of the Invention
The present invention relates to a zoom lens suitable for use in video cameras or electronic still cameras, wherein an image of a subject is formed on a solid state imaging device.
2. Background Arts
In video cameras and electronic still cameras, an image of a subject is formed on a photo-receptive surface of a solid state imaging device, wherein a matrix of micro-lenses are put on the photo-receptive surface, and an optical low-pass filter is usually placed in front of the photo-receptive surface. In order to improve photosensitivity of the solid state imaging device, it is important to prevent shading. For this purpose, light from the subject should fall onto the micro-lenses in a substantially perpendicular direction to the photo-receptive surface and thus to the micro-lenses.
For this reason, a retrofocus lens consisting of a front lens group having a negative power and a rear lens group having a positive power has conventionally been used as a zoom lens for forming an image on the solid state imaging device while changing its focal length from a wide-angle position to a telephoto position. It is to be noted that "front" means the object side and "rear" means the image side of any lens system in the following description. Since the retrofocus lens has a longer back focus or back focal distance than its focal length or front focal distance, and also the exit pupil is located distant from the lens, the light from the subject falls onto the micro-lenses substantially perpendicularly.
As for the retrofocus type zoom lens consisting of a negative front lens group and a negative rear lens group, the front lens group is moved together to the object side for focusing. According to this configuration, the lens must have a large diameter in order to obtain a sufficient brightness of the edge of image field while the zoom lens is focused on a near distance. As a result, the conventional retrofocus type zoom lens tends to be large in scale. Moreover, since it is necessary to move a plurality of lens elements of the front lens group together for focusing, a large driving system that can bear a large load for moving the plurality of lens is necessary, and it also enlarges variations in aberrations such as spherical aberrations, astigmatism and distortion.
To solve this problem, JPU No. 62-76312 discloses a retrofocus type zoom lens whose rear lens group is divided into a positive lens group having a positive power and a negative lens group having a negative power which are arranged in this order from the objective side, wherein either one of the positive and negative lens groups is moved for focusing. However, placing a negative lens in the rearmost position, i.e., the nearest position to the image side, of the lens system makes its exit pupil closer to the lens system. As such it is impossible to get a sufficient back focus. Also, the lens system as a whole would have a stronger positive power of refraction, making it hard to balance aberrations.